1. Field of Invention
This invention pertains to apparatus and method for producing semiconductor devices such as integrated circuits (e.g., IC, LSI) and active matrix type liquid crystal display devices, and particularly to apparatus and method involving plasma-assisted deposition.
2. Related Art and Other Considerations
MIS (Metal Insulator Semiconductor) devices (e.g., transistors) for integrated circuits (e.g., IC, LSI) and for active matrix type liquid crystal display devices (hereinafter, referred to as an AMLCD) have a metal for which voltage can be controlled, with the metal producing an electric field in the oxide/insulator, which field extends into the semiconductor. By varying of the electric field, conductive properties of the semiconductor can be varied.
MIS devices (e.g., transistors) can be produced by layering and then processing thin films such as a gate insulating film and a gate electrode on a semiconductor substrate. In many prior art techniques, for a semiconductor device using a silicon substrate or a liquid crystal display device using a quartz substrate, silicon was oxidized at a temperature of about 1000.degree. C. in an oxidizing atmosphere to form a silicon dioxide (SiO.sub.2) film as the gate insulating film.
In recent years, devices such as AMLCD panels have been produced by forming a polycrystalline silicon film on a glass substrate, primarily using atmospheric vapor phase deposition methods and plasma vapor phase deposition methods. Unfortunately, each of these methods suffers certain disadvantages including those mentioned below.
The atmospheric vapor phase deposition methods cannot reach a sufficiently high temperature, with the result that an insulating film with low density is obtained. In view of such low density, it is subsequently necessary to subject the insulating film, after deposition, to thermal annealing at 600.degree. C. for a long period of time.
In the plasma vapor phase deposition methods, exposing the surface of a semiconductor substrate to plasma can damage or contaminate the surface, as hereinafter explained. In the plasma vapor phase deposition methods, semiconductor devices are prepared in a plasma reactor wherein active species from a plasma react with wafer surface materials.
Conventional plasma reactors utilized for semiconductor fabrication are often parallel plate reactors having round plates. In most parallel plate reactors, the plasma is situated about 2 to 5 cm above a top surface of the sample, and the plasma is substantially coextensive with the sample top surface. Accordingly, in conventional parallel plate reactors, gas is excited to be decomposed by a plasma electric potential and simultaneously the decomposed gas is reacted with the surface of the substrate.
Other prior art plasma reactors have an essentially cylindrical tube shape wherein the plasma is further distanced from the sample top surface. Such reactors are accordingly known as remote plasma reactors or afterglow plasma reactors.
A constant challenge in semiconductor fabrication in plasma reactors is uniformity of processing over the entire sample. In a silane-based process, for example, such uniformity generally requires uniform distribution of silane in the reactor. To obtain uniform processing, it is important to uniformly couple energy to the plasma.
For inclusion in commercial products, semiconductor devices should maintain quality performance over useful life. In the case of thin film transistors (TFTs), for example, it is desirable to maintain a constant voltage threshold and gain over the useful life of the TFT. However, the presence of hydrogen at an interface of the insulator and semiconductor can jeopardize long-run performance of a MIS device such as a TFT.
In the above regard, when bonds between silicon and hydrogen are broken near an interface, the hydrogen essentially drifts away and its formerly bonded silicon becomes a trap for electrons which are tunneling from the semiconductor into the interface, thereby increasing threshold voltage and changing the transconductance of the TFT.
While the presence of hydrogen at an interface has thus been recognized as a potentially deleterious factor, prior art semiconductor processes have fortuitously minimized the influence of hydrogen. In this respect, prior art semiconductor processes typically have high temperatures (over 600 degrees centigrade for TFT fabrication for LCDs) which cause hydrogen to spontaneously desorb from the surface.
There is impetus to improve MIS production technology, such as (for example) utilizing less expensive materials and providing the MIS devices on larger silicon wafers. Concerning materials, rather expensive quartz or special glasses are necessary to withstand the high temperatures. While it is desirable to use less expensive glass (e.g, more common grades) for MIS substrates, the processing temperature must be significantly lowered to accommodate such lower grades, with the trade-off that hydrogen at the interface must somehow be otherwise removed (since temperature is no longer automatically causing hydrogen desorption).
Low temperature plasma reactive operations have previously been employed for relatively successful (e.g., relatively defectless) deposition of insulators on single crystal silicons. Significantly, MIS devices, on the other hand, are polycrystalline surfaces.
In one recent pseudo-low temperature prior art process, less expensive glass is utilized in a process of forming polycrystalline silicon by laser pulsing amorphous silicon. In such process, the peak temperature is very high, but because of its pulsed nature, the total thermal (local) load is low. However, such laser-involved process has considerable manufacturing complexity.
Other methods for forming thin films on a semiconductor substrate have recently been proposed. One method using an ECR plasma resonates electrons using a cyclotron to increase the density of plasma even at low pressure. In this ECR plasma method, a plasma chamber and a reaction chamber are separated from each other, so that excited species are produced in the plasma chamber and the resulting plasma is drawn into the reaction chamber (i.e., the surface of a semiconductor substrate). Such a structure prevents the surface of the semiconductor substrate from being exposed to the plasma, so that the surface is not likely to be damaged and excited species with a high density can be produced. Unfortunately, the method using the ECR plasma requires the confinement of the plasma with a magnetic field, which causes problems in forming uniform thin films over a large area.
Another method has been proposed in IEEE, EDL, Vol. 15, No. 2, February 1994. In this method, the damage to the surface of a semiconductor substrate caused by plasma is alleviated by interposing two other electrodes between electrodes in a parallel plate type plasma vapor phase deposition apparatus. Yet, problematically, SiH.sub.4 and O.sub.2 used for forming thin films on the substrate are simultaneously decomposed, so that the composition of the thin films is difficult to control and hydrogen cannot be prevented from being mixed in the thin films. Furthermore, this method makes it difficult to form uniform thin films over a two-dimensional large area.
In the unrelated field of production of architectural glass, glass has been conveyed past a linear source in a reactive sputtering process. However, reactive sputtering is not a chemical vapor deposition process and the architectural glass production technique moreover requires a rotary magnetron head.